Scientists using NASA's Swift satellite have spotted a stellar flare on a nearby star so powerful that, had it been from our sun, it would have triggered a mass extinction on Earth. The flare was perhaps the most energetic magnetic stellar explosion ever detected.

The flare was seen in December 2005 on a star slightly less massive than the sun, in a two-star system called II Pegasi in the constellation Pegasus. It was about a hundred million times more energetic than the sun's typical solar flare, releasing energy equivalent to about 50 million trillion atomic bombs.

Fortunately, our sun is now a stable star that doesn't produce such powerful flares. And II Pegasi is at a safe distance of about 135 light-years from Earth. 

Yet in detecting this brilliant flare, scientists obtained direct observational evidence that stellar flares on other stars involve particle acceleration, just like on our sun. Rachel Osten of University of Maryland and NASA Goddard Space Flight Center in Greenbelt, Md., presents this finding today at the Cool Stars 14 meeting in Pasadena, Calif.

"The flare was so powerful that, at first, we thought it was a star explosion," said Osten, a Hubble Fellow. "We know much about solar flares on the sun, but these are samples from just one star. This II Pegasi event was our first opportunity to study details of another star's flaring as if it were as close as our sun."

Solar flares on the sun originate in the corona, the outermost part of the sun's atmosphere. The corona's temperature is about two million degrees Fahrenheit, while the sun's surface, called the photosphere, is only about 6,000 degrees. The flare itself is a burst of radiation across much of the electromagnetic spectrum, from low-energy radio waves through high-energy X-rays. The X-ray emission can last up to a few minutes on the sun; on II Pegasi it lasted for several hours.

The flare involves a shower of electrons raining down from the corona onto the photosphere, heating the coronal gas to temperatures usually encountered only deep inside the sun. Scientists think that the twisting and breaking of magnetic field lines lacing through the corona generate the particle acceleration and flaring.

The flaring star in II Pegasi is 0.8 times the mass of the sun; its companion is 0.4 solar masses. The stars are close, only a few stellar radii apart. As a result, tidal forces cause both stars to spin quickly, rotating in step once in 7 days compared to the sun's 28-day rotation period. Fast rotation is conducive to strong stellar flares.

Young stars spin fast and flare more actively, and the early sun likely generated solar flares on par with II Pegasi. Yet II Pegasi could be at least a billion years older than our middle-aged 5-billion-year-old sun. "The tight binary orbit in II Pegasi acts as a fountain of youth, enabling older stars to spin and flare as strongly as young stars," said Steve Drake of NASA Goddard, a co-author with Osten on an upcoming Astrophysical Journal paper.

The key finding in the II Pegasi flare was the detection of higher-energy X-rays. Swift's Burst Alert Telescope usually detects gamma-ray bursts, the most powerful explosions known, which arise from star explosions and star mergers. The II Pegasi flare was energetic enough create a false alarm for a burst detection. Scientists quickly knew this was a different kind of event, however, when the flare overwhelmed Swift's X-ray Telescope, a second instrument.

Higher-energy "hard" X-ray detection in this case is the telltale signal of electron particle acceleration, creating what is called non-thermal X-rays. NASA's RHESSI mission sees this in the sun's solar flares. While lower-energy "soft" X-rays from thermal emission have been seen on other stars, scientists have never seen hard X-rays on any flaring star other than the sun. Because the hard X-rays occur earlier in the flare and are responsible for heating the coronal gas, they reveal unique information about the flare's initial stages.

Had the sun flared like II Pegasi, these hard X-rays would have overwhelmed the Earth's protective atmosphere, leading to significant climate change and mass extinction. Ironically, one theory posits that stellar particle outbursts are needed to condition dust to form into planets and perhaps life. The Swift observation demonstrates that such outbursts do occur.

"Swift was built to catch gamma-ray bursts, but we can use its speed to catch supernovae and now stellar flares," said Swift Project Scientist Neil Gehrels of NASA Goddard. "We can't predict when a flare will happen, but Swift can react quickly once it senses an event.

Osten's colleagues on this result also include Jack Tueller and Jay Cummings of NASA Goddard; Matteo Perri of the Italian Space Agency; and Alberto Moretti and Stefano Covino of the Italian National Institute for Astrophysics.